Tyrrell and Anna have stated my views better than I'd previously gone so far as verbalizing.

There are large sectors of the mind in which belief tends to become reality, including important things like "I am the sort of person who continues even in the face of adversity" and "I do have the willpower to pass up that cookie."

But - given that you aren't actually trying to fool yourself - there's a chicken-and-egg aspect that depends on your having enough potential in this area that you can legitimately believe the statement will become true if you believe it. At that point, you can believe it and then it will be true.

There's an interesting analogy here to Lob's Theorem which I haven't yet categorized as legitimate or fake.

To look at it another way, this sort of thing is useful for taking simultaneous steps of self-confidence and actual capability in cases where the two move in lockstep. Or, in the case of anti-competencies like doublethink, the reverse.

"I have the potential to be the sort of person who continues even in the face of adversity", or "it is more in my interests to pass up that cookie", or "I really do have a choice whether or not to pass up that cookie". That is what I would recommend.

bill, below, has mentioned "Act as if": "I choose to Act as If I can continue even in the face of adversity, and I intend in this precise moment to continue acting, even if I may just fall down again in two minutes' time".

These have the advantages of being more likely to be true.

Rambling on a little, to be the sort of person who continues in the face of adversity is Difficult, and requires practice, and that practice is very worthwhile. Stating that it is True might make you fail to do the practice, and instead beat yourself up when it appears not to be true.